The IAEA has the most extensive inspection program in place that's ever existed in history in Iran. The IAEA, under the agreement, has access to any place in Iran where any suspected violations could occur. The IAEA has certified compliance by Iran because the IAEA has the inspectors in place to verify compliance by Iran with the Joint Comprehensive Plan of Action (JCPOA) has been working perfectly in removing highly enriched (20%) uranium and closing down those centrifuges that produce this enriched uranium.

Iran was actually in compliance because it was beneficial economically for Iran to comply with the agreement. The UN Security Council got what it wanted (a verifiable means to ensure the end to the possibility of Iran producing a nuclear weapon) and Iran got what it wanted (lifting of sanctions against Iran that released frozen Iranian assets and allowing Iran to access international markets).

That's how diplomacy works. It creates a win-win situation where both sides want to comply with the negotiated agreement.

Even most of the initial opponents to the PCPOA have since admitted that it's working in preventing Iran from being able to produce a nuclear weapons.

__________________"I always had a rule, if a restaurant is dirty on the outside, it's dirty on the inside." Donald Trump

"I always had a rule, if the White House is dirty on the inside, it's dirty on the outside." ShivaTD

Based upon the corruption, brutality, inhumanity, immorality, dishonesty, and incompetence of the Trump administration the White House is the dirtiest house in America and there's no known cleanser that with remove the stains of the Trump Administration.